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Repair of Ultra-Thin Whitetopping
Ultra-thin whitetopping (UTW), a 2 to 4 inch (50-100 mm) concrete overlay on existing asphalt pavement, is a viable and successful pavement rehabilitation strategy for intersections, ramps, low-volume streets and highways, local roads, and light aircraft aprons. Since 1992, over 200 distressed asphalt pavements in at least 35 states have been rehabilitated with UTW. As UTW pavements age and carry traffic repair may eventually be required. UTW pavements may also require cuts for access to underground utilities.
Since UTW pavements are 2 to 4 inches (50-100 mm) thick, any repairs made are usually full replacement of the concrete, down to the underlying asphalt. The repair process is much like that of full- depth repair of traditional concrete pavement, except that no load transfer devices (smooth dowel bars) are needed.
When to Repair UTW Roadways
Performance studies show that cracking alone does not warrant UTW repairs. Even with eventual development of corner, longitudinal, or transverse cracking, most UTW pavements will continue to provide good service for many years. The presence of fibers in the concrete, a common UTW practice, helps keep the concrete intact after cracks appear. Generally, repairs should be considered when:
- Panels are broken into four or more pieces
- Surface irregularities or settlement affect ride quality
- Loose or missing concrete is evident
The repair of UTW pavements involves locating and replacing deteriorated panels. Because UTW joint spacing usually creates small panels, the removal and replacement of the concrete is very simple.
There are six steps involved in repairing a UTW pavement:
- Identify the panels to be removed
- Saw cut the perimeter to full depth of concrete
- Remove the concrete panels and any deteriorated asphalt
- Prepare the patch area
- Place, finish, and cure the new concrete
- Saw joints and open to traffic
Identify and Isolate Slabs to be Removed
The first step in the repair process for UTW is to determine which panels need to be replaced. A survey is usually conducted by the agency responsible for the maintenance of the pavement. The panels are usually identified with bright-colored paint. Cracks in panels do not require repair, but if the UTW overlay breaks up and begins moving or dislodging, repairs are warranted. The entire panel containing the distress should be removed and replaced; partial panel replacement is not recommended.
Saw Cut Panels to Bottom of Concrete Overlay
The distressed panel should be cut at all joint lines using diamond or abrasive-bladed saws to the bottom of the concrete overlay. Care should be taken not to damage the underlying asphalt layer. In many cases, a second saw cut about six inches inside the panel joint is helpful in avoiding damage to adjacent panels during the removal process. Full-depth sawing to the bottom of the concrete overlay results in clean cuts, easy removal of the deteriorated concrete, and less joint spalling.
There are two ways to remove the deteriorated concrete:
- Break up
- Full panel removal
To break up the concrete, jackhammers are used (30 lb. maximum for interior concrete; 15 lb. recommended near joints and repair boundaries). Once the first panel is broken up with jackhammers and removed, a small front-end loader can be used to pick up large sections, if the bond between the asphalt and concrete is not too strong. Care must be taken not to damage the adjacent panels with any equipment during the removal process.
Prepare Patch Area
The broken pieces of concrete must be removed to prepare for the placement of new concrete. Next, the stability of the underlying asphalt should be evaluated, and unsound material removed. If granular base material is exposed or if the underlying asphalt is less than 3 inches (75mm), a full-depth concrete patch (minimum 6-8 inches or 150-200 mm thick) is recommended.
Before concrete placement, the asphalt surface should be cleaned by air blasting with clean, compressed air. Sand or shot blasting is required if air blasting will not remove foreign material from the surface of the asphalt. The asphalt surface must be kept clean prior to the placement of new concrete.
Place New Concrete
The placement of new concrete into the patch areas of the UTW pavement should follow these 6 steps:
- Place the concrete into repair areas. The concrete mixture should be at the same proportions as the existing concrete.
- Consolidate the concrete using a handheld vibrator
- Finish with a straight edge or vibratory screed to meet the existing grade/profile
- Texture the concrete surface (i.e. burlap drag) to match the surrounding panels
- Apply curing compound immediately after the bleed water sheen disappears
- Cover with insulating blankets if the ambient temperature falls below about 40°F (5°C) within 24 hours of placement or as required for early concrete strength for opening to traffic.
Saw Joints and Open to Traffic
Saw joints as soon as possible without raveling the concrete to match the width and location of existing and adjacent joints. Typically, joints in UTW are 1/8 in. (3mm) wide, and to a depth of 1/3 of the UTW thickness.
The repaired pavement can be opened to traffic once the concrete has reached the specified opening strength, maturity, or curing time.
Ultra-thin whitetopping repair is not difficult. When done properly, UTW repairs are efficient, long lasting, and cost effective. With proper design and construction techniques, UTW patches significantly extend the performance life of UTW pavements.
Generally, UTW repairs are made on individual panels by replacing the full thickness of the concrete overlay. Cracking alone does not warrant repairs. Panels should be replaced when ride quality is suffering, when loose or missing concrete is evident, or when underground utility repairs are made.
For more information about UTW, contact your local concrete paving representative, visit ACPA’s website www.pavement.com, and see the American Concrete Pavement Association’s publica- tions "Ultra-Thin Whitetopping" (IS100.02P), "Construction Specification Guideline for Ultra-Thin Whitetopping" (IS120P), and "Whitetopping—State of the Practice" (EB210.02P).
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